Zone heating for respiratory circuits

A respiratory humidification system comprising:

• an inspiratory limb including a first segment with a first heater wire, a second segment with a second heater wire, and a temperature sensor positioned at a patient end of the second segment for measuring a patient end parameter, wherein the first and second heater wires are electrically coupled, the first heater wire forming a first heater circuit and the first and second heater wires forming a second heater circuit, the first and second heater wires configured to heat respiratory gases passing through the inspiratory limb;
• and
• a flow sensor in a flow path of the system and configured to measure a flow rate of the respiratory gases;
• a controller in electrical communication with the first and second heater wires and the temperature and flow sensors and configured to control the first and second heater circuits, wherein, when a difference between an output of the temperature sensor and a patient end parameter set point is below a predetermined error threshold, the controller is configured to heat the respiratory gases using the first heater circuit until a maximum temperature is reached in the first heater wire, and when the difference between the output of the temperature sensor and the patient end parameter set point is at or above the predetermined error threshold, the controller is configured to heat the respiratory gases using the second heater circuit, and
• wherein when a measured flow rate is at or below a predetermined flow rate threshold, the controller is configured to control a duty cycle or power supplied to one or both of the first heater circuit or the second heater circuit using a first heater circuit low flow cap or limit and/or a second heater circuit low flow cap or limit to prevent a surface temperature of a portion of the first segment of the inspiratory limb adjacent the second segment of the inspiratory limb from exceeding a surface temperature threshold.